1. Field of the Invention
The present invention relates to carbon materials, and particularly to nitrogen and phosphorus co-doped crystalline carbon materials that are synthesized by a facile, template-free method and exhibit high crystallinity.
2. Description of the Related Art
Carbon exists in three major forms, namely, graphite, amorphous carbon, and diamond. Each of these three forms of carbon varies in structure and bonding, specifically with respect to the carbon-carbon bond. Individually, the amorphous carbon has no crystal structure and is often considered as an impure form of carbon when compared with graphite or diamond. The typical examples of amorphous carbon include, for example, coal and soot. Graphite is a native crystalline form of carbon, which naturally exists in metamorphic and igneous rocks. Graphite is a soft, black, and highly pure form of carbon. Structurally, it consists of sheets of covalently bonded carbon atoms in layers that are held together with weak van der Waals interactions between the carbon planes. The third major polymorph of carbon is diamond. Diamond is composed of carbon atoms bonded to each other in a three dimensional face-centered crystalline cubic structure.
In addition to these three forms of carbon, buckminster-fullerene (C60), single sheets of graphite (known as graphene), and nano forms of carbon, such as single wall carbon nanotubes (CNTs) or multiwall carbon nanotubes (MWCNTs), are also polymorphs of carbon that are widely used in a number of applications. Activated carbon, another carbon polymorph, is a crude form of graphite with an amorphous structure, which has a very high surface area and also has a wide range of industrial applications.
Recently, it has been found that doping carbon with a heteroatom, e.g., nitrogen (N), Sulfur (S), Phosphorus (P), and/or Boron (B), can significantly improve the performance of resulting Carbon Materials (CMs). Various such materials can be used in a wide range of applications in environmental treatment (CO2 adsorption), catalysis, and electrochemistry. In particular, nitrogen-doped CMs are receiving growing attention due to their excellent catalytic performance for Oxygen Reduction Reactions (ORR), which is one of the most important energy-consuming and kinetically less favorable reactions in the power generation mode of a fuel cell. More recently, it has been reported that some of the heteroatom-doped carbon materials, for example, Nitrogen-doped CM, Nitrogen- and Oxygen-doped CM, and Nitrogen- and Phosphorus-doped CM, exhibited facile catalytic activities of water oxidation/Oxygen Evolution Reaction (OER). The heteroatom-doped carbon materials have also been explored for electrochemical energy storage devices, such as secondary batteries (rechargeable) and supercapacitors.
In order to make the best use of CMs for advanced energy applications (energy conversion and energy storage), much effort has been focused on the synthesis of desired heteroatom-doped CMs with controlled structure and texture, morphology, high surface area, high level of graphitization, large pore volume, and doping CM with desired heteroatom(s) (single or co-doped CM). However, except for a few new carbon sources, most of the currently available starting precursors for CM synthesis are limited to only a few conventional sources, such as naturally occurring carbon sources (wood, lignite, coal and some other petroleum based materials), cellulosic materials, and a few synthetic polymeric materials. For example, human hair has been successfully used as a natural precursor to synthesize nitrogen and sulfur co-doped CMs with high surface area, high electrical conductivity, and facile catalytic efficiency for sluggish ORRs in fuel cells. Similar to human hair, chicken feather, which is a biopolymer waste from the poultry industry, has also been successfully processed to synthesize nitrogen-doped carbon materials.
The major advantage of biopolymer-based starting precursors is the high concentration of intrinsic nitrogen and carbon content, which upon annealing, produces nitrogen-doped carbon materials in high yields. In order to achieve the desired concentration of heteroatom-doped carbon materials with desired functionalities, surface characteristics, and potential applications, a number of organic compounds, such as organic ionic dyes, organic ionic salts, and simple organic precursors (added with some nitrogen, phosphorus or sulfur dopant) have been explored by simple carbonization in a controlled environment to produce heteroatom-doped carbon materials. However, synthesizing heteroatom-doped CMs from each of these precursors, and the respective methods used to process them, have their own constraints and limitations.
To date, most of the attempts to synthesize heteroatom-doped carbon materials remain focused on exploiting new starting precursor materials and the properties of heteroatom-doped carbon materials. Therefore, heteroatom-doped carbon materials are typically formed entirely dependent on the nature of the starting precursor and carbonization temperature. In particular, some of the important characteristics of heteroatom-doped carbon materials, such as surface area, porosity, degree of graphitization, and concentration of dopant, heavily depend on the carbonization temperature to a large extent. Also, the structural characteristics, such as crystalline or amorphous nature of the heteroatom-doped carbon material, show a high dependency on the carbonization temperature in most of the previous reports.
Many of the existing methods of heteroatom doping of crystalline carbon materials use either a hard template (such as silica) or soft templates (eutectic mixtures of metal salts) to obtain the crystalline phase. However, it is very difficult to completely remove the template. Some studies have shown that crystalline carbon materials prepared using soft templates still contain a residue of metal ions, which may alter structural and electronic properties of the active sites of these carbon materials. Since both hard and soft templates can reside in ppm levels in the final doped product, there is a need for a facile, template-free process for synthesizing heteroatom-doped crystalline carbon materials.
Thus, nitrogen and phosphorus co-doped crystalline carbon materials solving the aforementioned problems are desired.